Semiconductor integrated circuits (ICs) have evolved towards increased density and device shrinkage. One important structure in the manufacture of ICs is isolation structures. Devices formed in the silicon substrate must be isolated from one another. Establishing effective isolation in submicron ICs in the face of decreased isolation space is a complicated and challenging task. In ULSI, a tiny amount of leakage per device due to improper isolation can induce significant power dissipation for the entire circuit.
A common way for isolation is the use of LOCOS (LOcal Oxidation of Silicon) isolation. LOCOS involves the formation of Field Oxides (FOX) in the nonactive regions of the substrate. As device geometry reaches submicron size however, conventional LOCOS isolation has its limitations. For example, the "bird's beak" structure of LOCOS causes unacceptably large encroachment of the field oxide into the device active regions. Further, the planarity (or lack thereof) of the surface topography after LOCOS is inadequate for submicron lithography needs. Therefore, trench isolation is one of the newer approaches adopted for use in isolation.
Trench isolation is used primarily for isolating devices in VLSI and ULSI, and hence they can be considered as replacement for conventional LOCOS isolation. Further, shallow trench isolation is gaining popularity for 0.25 micron technology. Within the dielectric isolation area, dielectric is refilled into a trench. In the basic shallow trench isolation (STI) technology, shallow trenches are anisotropically etched into the silicon substrate. Next, a CVD oxide is deposited onto the substrate and is then planarized by CMP (Chemical Mechanical Polishing) or etching back. Unfortunately, the planarization of shallow trench isolation relies on chemical mechanical polishing (CMP) which has been proven an effective but challenging process.
The aforementioned challenges associated with CMP for STI include the "dishing effect" for wide trenches. The dishing effect degrades the planarity of a layer, and it also impacts the control of implantation. Some prior art attempts to solve the problem use reverse tone structure. This method involves forming protruded portions to eliminate the dishing effect. However, the conventional method needs a photomasking step, an etching process and a step to remove the photomasking.
What is needed is a more efficient method for forming high quality shallow trench isolation structures.